Due to good stress-relaxation, electrical, heat-resistant, and weather proof properties of cured compounds produced from silicone gel compositions, these compositions find wide application for preparation of fillers and sealants used in electric and electronic devices. An example of such a silicone gel composition is the one having a branch-structured polyorganosiloxane having a molecular terminal capped with a silicon-bonded vinyl group, a polydiorganosiloxane having both molecular terminals capped with silicon-bonded vinyl groups, a polyorganosiloxane having silicon-bonded hydrogen atoms only on both molecular terminals, and a platinum catalyst (see Japanese Patent Application Publication No. Sho 62-181357).
However, a silicone gel obtained by curing the aforementioned composition has poor adhesion to a substrate and can be easily peeled off from the substrate under the effect of heat cycles and physical stress. It has been proposed to overcome the above drawback by using a silicone gel composition improved either by adding siloxane units having alkoxy or epoxy groups to the base polymer or to a cross-linking agent (see Japanese Patent Application Publication No. Hei 4-88060). Another method is based on the use of a silicone gel composition based on the addition of an aluminum organic compound and an alkoxyalkyl silane compound to a specific polyorganosiloxane compound (see Japanese Patent Application Publication No. Hei 6-107947), or on the use of a polyorganosiloxane composition that contains a chain extender having in one molecule two silicon-bonded hydrogen atoms, a cross-linking agent with at least three silicon-bonded hydrogen atoms in one molecule, an alkylpolysilicate, and an alkyl titanate (see Japanese Patent Application Publication No. Hei 7-233326).
However, improvement achieved in thermal stability and in adhesion of a silicone gel obtained by curing the above composition to a substrate is not sufficient, and when such silicone gel is maintained over a long period of time at relatively high temperatures that exceed 180° C., it begins to lose its consistency, i.e., becomes harder, can easily be peeled off the substrate, and forms “cracks” in the gel. Furthermore, if a silicone gel composition contains an organoaluminum compound, long-term storage of this composition prior to curing causes precipitation and separation of the organoaluminum compound.